Scintillating counters are important tools for studying nuclear radiation. Nuclear radiation can be made up of energetic particles possessing mass and charge such as alpha and beta radiation. Radiation can also be in the form of particles having mass and no charge such as neutrons, or as particles with no mass or charge such as gamma rays. All of these forms of radiation interact with matter at the atomic level.
A scintillation counter consists of a scintillation detector usually in the form of a solid or liquid material which produces a flash of light upon excitation by incident radiation. The scintillating material is optically coupled to a photomultiplier tube which converts the photons to electrical pulses whose magnitude is proportional to the intensity of the initial light flashes. This, in turn, is proportional to the toal energy dissipated by the radiation energy in the detector. The photomultiplier pulses may be measured to provide information on the impinging radiator.
In the case of alpha, beta and gamma radiation, interaction with matter results in disturbing the electron level of an atom with which it comes in contact. Electrons only slightly disturbed can be raised to higher energy levels. Electrons involved in direct interaction can be released from their electronic field with high kinetic energy, resulting in the formation of an ion pair. These high energy electrons dissipate their energy by interaction and excitation of other electrons of neighboring atoms. The net result is that the disturbed and released electrons return to ground state or normal energy levels and release their kinetic energy as photons. The electrons produced from the total capture of an incident nuclear radiation within a crystal detector are referred to as photoelectrons. They represent the highest level of response of the detector. Radiation energy only partially retained by the detector results in the production of lower energy electrons called Compton electrons. The energy of these electrons is lower than that of photoelectrons and ranges over a complete energy spectrum.
A great deal of work has gone into finding good scintillating compounds of which the superior ones are more generally referred to as phosphors. The physical properties which are most often associated with phosphors are: (1) the ability to convert a large fraction of the incoming radiation to excited energy which is released as photon energy, (2) emitted photon energy possessing a wavelength of maximum spectral sensitivity to the phototube, (3) photon emission of short duration to allow discrete radiation energy to be measured at high incident intensity, and (4) a phosphor having good optical properties to permit the photon energy to be transmitted. Since no one phosphor has all of these properties, one must choose a phosphor for the type of radiation to be detected.
Scintillators may be inorganic or organic in nature. Inorganic scintillation crystals are well known and include CaWO.sub.4, MgWO.sub.4, LiAlSi.sub.2 O.sub.6, LiF, CdB.sub.4 O.sub.7, ZnO, CdS-Cu, CdS-Ag, ZnS-Cu, and ZnS-Ag. The inorganic scintillation crystals which are presently most used in nuclear radiation detectors are the alkali metal halide crystals activated by the inclusion of thallium. These crystals are preferred herein. Such crystals are NaI(Tl), NaBr(Tl), KBr(Tl), KI(Tl), KCl(Tl) and the like with NaI(Tl) preferred. The superior alkali metal iodide crystal phosphors used today contain approximately 10.sup.-.sup.1 mol percent of thallium as an impurity in the crystal lattice. It is believed that thallium in the crystal lattice acts as an impurity center which may be raised to an excited state either by absorption of photons, by capture of an exciton (defined as an electron hole in the crystal lattice) or by the successive capture of an electron and a hole. The importance of the thallium as an activator center is that it permits the excited energy to transcend otherwise forbidden energy levels of the crystal to the allowed levels which favor the scintillation process by photon emission during energy decay to ground state levels.
The scintillation process in organic liquid scintillators and solid organic compounds, known as plastic scintillators, is primarily a molecular response. As such, it is distinguished from inorganic solids whose luminescence is intimately associated with energy-bond structure of ionic crystals. Organic solids, i.e., plastic scintillators, are classified as molecular crystals where the intermolecular bonding is quite weak (Van der Waal forces) compared with bonding of ionic crystals. Photoluminescence in organic scintillators arises from de-excitation of the first excited electronic state. This excitation can occur when radiation energy excites and distorts the electronic cloud associated with an organic molecule. Molecules of cyclic nature such as benzene and benzene ring-type compounds exhibit excellent organic scintillation properties because they are easily excited due to their already high resonating energy state.
The choice of an inorganic crystal or an organic phosphor for nuclear radiation measurement by scintillation detectors thus depends on the type of radiation energy one wishes to measure and to what use the radiation measurements will be applied. In the measurement of gamma radiation one usually selects an inorganic crystal detector such as sodium iodide thallium activated crystals. There are two very important reasons for this selection. The first is that gamma radiation which possess no mass or charge but discrete energy is not highly interactive with low density compounds such as organic phosphors, but is more efficiently detected with the high density inorganic crystals. Another way of stating this would be to say that gamma rays are not reactive with the low mass carbon and hydrogen atoms of organic phosphors but are reactive with the higher mass sodium and especially iodide ions in the crystal detectors. A second important feature about the inorganic crystal phosphor detectors and one that relates to their high efficiency for gamma ray interaction is that they permit total capture of many of the incident gamma rays which enables identification of the omitting radioisotopes. Therefore, the use of an inorganic crystal not only acts as an efficient detector but offers a means for identifying the source of gamma radiation through radiation energy measurement by total gamma ray capture (gamma ray spectroscopy).
Organic phosphor detectors have been used mainly for radiation detection of charged particles. Alpha and beta radiation having mass and charge are very interactive with low mass organic phosphors and can be effectively detected with even small volumes of organic phosphors. The stress for determining the energy level of incident radiation as stated for gamma ray spectroscopy is not as important for radiation energy such as beta particles which are not released at discrete energy levels but rather over a wide energy spectrum. Organic phosphors find their greatest use where phosphor detectors are not needed specifically for radiation energy measurement, but are used for charged particle detection and where detector configuration and cost are prime considerations in the scintillation system.
In order to achieve maximum density, optical transparency and uniform activation, it has been necessary heretofore to prepare the scintillator as a single crystal. Hence, probably the biggest drawback to the use of inorganic crystal detectors is their high cost, and the limited configuration and size to which the crystal detectors can be fabricated. The main reason for the high cost is that previous to this invention, a high efficiency and energy resolution detector (one that has high capability for capturing incident gamma radiation and measuring the total radiation energy) required a single crystal to be grown and fabricated into a detector. While large crystals have been grown and fabricated (&gt;4,000cc) and cut to various sizes and shapes, their costs have been prohibitively high, and their detector configuration limited.
Attempts have been made in the past to utilize small quantities of broken or small crystals to overcome the difficulties and expense encountered in growing large, pure crystals. Thus, an article by R. D. Albert on page 1,096 in The Review of Scientific Instruments, Vol. 24, Dec., 1953, describes endeavors to use clusters of scintillation crystals as radiation detectors. The crystal pieces, composed of NaI(Tl) were immersed in a mineral oil so as to light couple the crystal pieces to each other for efficient photon transfer. The mineral oil was chosen with a like refractive index to that of the crystals and good optical transfer properties for the emitted photons. The crystal detector was not used for energy measurements as in gamma ray spectroscopy because it had very poor energy resolution capabilities for resolving and measuring the total capture radiation. This failing was due to the multiresponse of each of the crystals to the radiation. Failing to give a single composite response as is obtained in a single large crystal, the detector was used only as an anticoincidence shield to detect incident gamma radiation requiring no energy resolution measurements.
Another form of multicrystalline scintillator is described by Tarmer and Derstein in Nucleonics, 10, 1952. A multicrystalline mass was prepared by melting crushed crystals of stilbene in an aluminum mold and allowing the mold to cool. The advantage of this type of nuclear detector fabrication was that it was very inexpensive and allowed irregular configuration detectors to be quickly constructed. It, however, had the drawbacks of all organic detectors. Its low mass made it a poor detector for noncharged radiation such as gamma rays and an even poorer detector for making energy resolution measurements.
A recent attempt has been made to construct a multicrystalline nuclear detector called Polyscin I which uses NaI(Tl) crystal chips or pieces. This work has been directed to constructing a nuclear detector that would not only detect nuclear radiation but would also measure the energy of the incident radiation. The detector is reported to be made of NaI(Tl) chips of rigid uniform size (range mm) with no solvent added. The advantage of the detector is its inexpensive construction and the feedom to fabricate it to any irregular size. The principle under which the detector apparently works is that with a rigid control and construction of like size dimensioned crystals the energy response for each crystal is equal and uniform and therefore additive. While this detector system represents an improvement and does possess some energy resolution capabilities compared to previously reported multicrystalline scintillation detectors, its performance is far less than the response of a single crystal detector of equal size and volume.
As has been descrbied heretofore, it has not been possible to use multi-inorganic scintillation crystal pieces to advantage as a single crystal detector because energy detection and resolution vary for each crystal piece resulting in a non-additive response for each small crystal. What occurs is that each small single crystal produces Compton and photoelectric responses to the incident radiation. As a result, the spectrum of energy detected represents many Compton and photoelectric peaks which are nondiscernible. From such spectra it is not possible to make energy calculations as can be done when a large single high efficiency scintillation crystal is used.